Many tumors in primary culture, both murine and human, cannot utilize cystine as a source of cysteine for growth. Attempts were therefore made to maintain an oxidative stress in vivo which would convert available cysteine to cystine and thus inhibit tumor growth. The mixture of hydroxocobalamin (vit. B-12) and ascorbate were very effective in oxidizing cysteine in vitro and were shown to increase the life span of P388 lymphoma-bearing mice by 70 percent. Cells which will not grow with cystine as a source of cysteine require mercaptoethanol or its oxidized form, hydroxyethyl disulfide, as a growth factor. These facilitate te uptake by the cell of the cysteine moiety as the mixed disulfice. A similar interaction with inverse purpose, the promotion of cytotoxicty of homocystine, selenocystine and selenocystamine by dimethyl disulfide was discovered. This reaction increased the cytotoxicity of selenocystamine in vitro by two orders of magnitude while reducing its host toxicity in vivo.